A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate. However, deposition is not restricted to the surface of the substrate, and this can result, for example, in the clogging of gas nozzles and the clouding of chamber windows. In addition, particulates may be formed, which can fall on the substrate and cause a defect in the deposited thin film, or interfere with the mechanical operation of the deposition system. As a result of this, the inside surface of the process chamber is regularly cleaned to remove the unwanted deposition material from the chamber.
One method of cleaning the chamber is to supply molecular fluorine (F2) to react with the unwanted deposition material. Fluorine is either supplied at a high purity (at least 99% fluorine) in single, high-pressure cylinder at 200 or 400 psig, or supplied diluted with nitrogen in the ratio of 20% F2 to 80% N2 in single, high-pressure cylinder at up to 2000 psig. At such high pressures, fluorine becomes very reactive and thus poses a serious safety hazard. While on-site fluorine generators are available for supplying high purity fluorine directly to the processing system, multiple generators are required to maintain fluorine output in the event of a failure of one of the generators, which significantly increases costs.
It is an aim of at least the preferred embodiments of the present invention to provide a low cost, relatively safe method of delivering high purity fluorine.